The present invention relates to an intake system for an internal combustion engine for controlling the amount of intake air by varying the valve lift characteristics of an intake valve.
In an internal combustion engine having a throttle valve in an intake passage for controlling the amount of fuel supplied to the engine in accordance with the amount of intake air by varying the opening degree of the throttle valve and thereby varying the amount of intake air, the intake vacuum downstream of the throttle valve becomes larger as the amount of intake air becomes smaller.
In the meantime, “larger vacuum” is herein intended to indicate a state that is higher in the degree of vacuum and closer to a complete vacuum, and “smaller vacuum” is intended to indicate a state that is lower in the degree of vacuum and closer to the atmospheric pressure.
On the other hand, in an in-cylinder injection type internal combustion engine in which fuel is injected directly into an combustion chamber to execute stratified combustion, the amount of injected fuel can be determined irrespective of the amount of intake air, thus making it possible to make smaller the intake vacuum irrespective of the operating condition of the internal combustion engine and thereby decrease the pumping loss.
Japanese Patent Provisional Publication No. 11-229928 discloses an example of setting or determination of the intake vacuum in the in-cylinder injection type internal combustion engine, i.e., to set the intake vacuum at a constant value a little smaller than the atmospheric pressure or to allow the intake vacuum to have such a variable characteristic of decreasing slightly with increase of the torque (refer to FIG. 11 of the above-described publication).